[GRASS-dev] Addons: Python error when checking if package is installed - r.randomforest

[Vaclav Petras]

On Mon, Jun 27, 2016 at 7:18 AM, Markus Neteler <neteler at osgeo.org> wrote:

> (I noted that various GRASS scripts use different ways for this check,
> the test for 3rd party packages should be standardized.)
>


I so far promoted lazy import* as a solution to these dependencies. The
important part is that import happens after calling grass.script.parser(),
so for manual and GUI, you don't need to have the dependencies at all. For
simple modules (with dependency usage limited to main() function), the
import** is done in main() after parser() call and try-except with
ImportError is used to catch the missing dependency and report an error to
the user (example: r.colors.cubehelix***). More complicated modules (with
explicit dependency usage outside of main() function) must be solved on a
case-to-case basis (examples: v.class.ml and v.class.mlpy).

Older discussions:

https://lists.osgeo.org/pipermail/grass-dev/2015-February/073734.html
https://lists.osgeo.org/pipermail/grass-dev/2016-March/079610.html

Examples:

https://trac.osgeo.org/grass/browser/grass-addons/grass7/raster/r.colors.cubehelix/r.colors.cubehelix.py#L186
https://trac.osgeo.org/grass/browser/grass-addons/grass7/raster/r.flexure/r.flexure.py?rev=64452
https://trac.osgeo.org/grass/changeset/66482/

Vaclav


* By lazy import I mean an import which is not done at the beginning of the
file, but somewhere in some function only right before it is actually
needed.

** I think that import with try-except is just more straightforward than
testing the presence of the module/package before the import. It also
follows "it is easier to ask for forgiveness than for permission" (EAFP),
so one can say that it is more Pythonic.

*** r.colors.cubehelix is actually not the basic example, because it
provides fallback when the dependency is not available and shows just
warning and an error but without exiting (uses warning() and error() and it
does not use not fatal()).
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